As of September 2014, my experience with zBrush is pretty limited. I touched the application once before while working with Team Ataxia last semester.
The tree asset was unique because of its really smooth, really curvy silhouette. While I was able to build the leaf "domes" in Maya, I assumed zBrush would be better equipped for molding and smoothing a trunk. So, I picked up the program to accomplish this.
Luckily, the program was intuitive enough for me to slowly maneuver the menus -- thanks to a little tutorial digging -- and I finished up with only some minor difficulty. I made one object which I edited with Maya, stretching and smoothing the trunk to get those varied twists.
Since my focus is with character design, I took zBrush for another spin during the first week of class to re-familiarize myself with the program. I wanted an opportunity to explore the program myself and get my own feel of things, so I tried not to rely on tutorials outside of very basic tool functions.
I already succeeded with building a very simple tree trunk, so I wanted to challenge myself by learning how to build, unwrap and paint a head. The biggest challenge was working with so many faces on an object -- especially with unwrapping -- which I solved by giving the model less subdivisions and smoothing later after importing the head to Maya.
My goal for this class is to become less intimidated by zBrush. Now that I've fiddled with the program a little more out of class, I think I can approach the program with much more confidence.